1. Field of the Invention
This invention relates to a beam splitter, and more particularly to a beam splitter comprising a multilayer interference film wherein the ratio of polarized light components to each other in the reflected light and the transmitted light can be made equal to the ratio of polarized light components contained in the incident light to each other, and can also be adjusted as desired.
2. Description of the Prior Art
In general, when unpolarized light obliquely impinges upon an interface of two different media, the component ratio of the P polarized light (i.e. the polarized light vibrating in parallel with the incident face) to the S polarized light (i.e. the polarized light vibrating perpendicularly to the incident face) which are contained in the light reflected from the interface becomes different from the component ratio of the P polarized light to the S polarized light which are contained in the light passing through the interface. The component ratio depends on the angle of incidence and the refractive indices of the two media. Normally, however, the reflected light contains the S polarized light in a proportion higher than the P polarized light, and the transmitted light contains the P polarized light in a proportion higher than the S polarized light. It is known that, in the Brewster angle condition at the interface of the two media, the S polarized light component and the P polarized light component can be approximately isolated from each other as the reflected light and the transmitted light, respectively. A polarizing prism utilizing such a phenomenon has been proposed, for example, in Japanese Patent Publication No. 55(1980)-9683, and has been put into practice in the MCA type photo-disk reproducing optical system of Philips, or the like. However, the polarizing prism used in the photo-disk reproducing optical system is not of the type essential as a basic element for reproducing the signals, but is used to stabilize the output of the laser generator serving as a light source for reproduction. Namely, in the conventional laser generator used for reproducing with the photo-disk, when reverse incidence of signal light called "back talk" occurs, the output light generated by the laser generator fluctuates depending on the phase of the back talk, and adversely affects the signal reproducing. To prevent the back talk, the aforesaid polarizing prism is used in combination with a quarter-wave plate in the conventional photo-disk reproducing optical system. (The functions of the polarizing prism and the quarter-wave plate, and details of the reproducing optical system are well known and not explained herein in further detail.)
More recently, various improved laser generators, for example, laser generators unaffected by the aforesaid back talk, have been proposed. It has also been proposed to detect signals by utilizing the reverse incidence itself by use of the self-coupling effect as in the case of a certain kind of semiconductor laser beam. Under these circumstances, the polarizing prism and the quarter-wave plate used in the conventional photo-disk reproducing optical system becomes unnecessary, and the construction of the reproducing optical system can be markedly simplified.
FIGS. 1 and 14 show examples of the photo-disk reproducing optical system having a construction simplified as described above. In each of FIGS. 1 and 14, a laser generator 1 is of the type unaffected by the back talk. Light emitted from the laser generator 1 is collimated into a parallel light beam by a collimator lens 2, and is then made to impinge upon a beam splitter 3. The beam splitter 3 is provided with a semi-transparent mirror 3' which may, for example, be of the 50% transmitting and 50% reflecting type. The light beam reflected from the semi-transparent mirror 3' is converged on the signal section of a photo-disk 5 by an objective lens 4, thereby to irradiate spot-wise the signal section provided with pits. The light reflected from the signal section is phase-modulated according to the shape and dimensions of the pit, and involves a change in intensity due to light interference. The phase-modulated light again impinges on the objective lens 4 as signal light. The signal light collimated into a parallel light beam by the objective lens 4 again reaches the semi-transparent mirror 3', and the light beam passing through the semi-transparent mirror 3' is detected by a photo-sensor 6. In the system described above, the light emitted from the laser generator 1 is once reflected from the semi-transparent mirror 3' and then transmitted therethrough. Therefore, when the transmittance (or the reflectance) of the semi-transparent mirror 3' is 50%, the amount of light reaching the photo-sensor 6 reduces to 25%. The light use efficiency obtainable in this case is the highest in the system described above.
In the optical system described above, the semi-transparent mirror 3' is used not only as the reflecting face but also as the transmitting face. Therefore, when a semi-transparent mirror of the type generally used is positioned in the optical system, the light use efficiency is further decreased for the reason as described below. Namely, as mentioned above, the reflecting and transmitting characteristics of an ordinary semi-transparent mirror differ between the P polarized light and the S polarized light. Accordingly, when the semi-transparent mirror first serves as the reflecting face, a major proportion of the P polarized light is transmitted therethrough, and the light mainly containing the S polarized light is reflected toward the photo-disk 5. (When the reflectance of the semi-transparent mirror is 50%, the total amount of the P polarized light and the S polarized light reflected is 50%.) Thereafter, when the signal light mainly containing the S polarized light and reflected from the photo-disk 5 again impinges upon the semi-transparent mirror 3', most of the signal light is again reflected from the semi-transparent mirror 3' because of the characteristics of the ordinary semi-transparent mirror. As a result, the light reaching the photo-sensor 6 becomes very weak, requiring a photo-sensor having a high capacity. To solve this problem, there is needed a semi-transparent mirror exhibiting the characteristics of approximately equally reflecting (or transmitting) the P polarized light component and the S polarized light component.
Further, in a photometric optical system of a camera, or the like, luminance detection is conducted via a semi-transparent mirror. In such an optical system, one portion of light split by the semi-transparent mirror is used for viewing through the view finder, and the other portion of light is used for photometry. In this case, most of the light impinging upon the semi-transparent mirror is the light reflected from the object region. Accordingly, the incident light is apt to contain mainly the S polarized light component. Particularly, when there is a reflective surface such as water surface or window glass surface in the object region, the incident light contains very much S polarized light component, depending on the angle of incidence. However, since an ordinary semi-transparent mirror reflects mainly the S polarized light, the amounts of light portions split by the semi-transparent mirror become unbalanced even when the transmittance of the semi-transparent mirror is 50%. Therefore, it is not always possible to correctly conduct the photometry. Also to solve this problem, a need exists for a semi-transparent mirror exhibiting equal reflecting and transmitting characteristics both for the P polarized light component and the S polarized light component.